A number of vascular diseases, such as coronary artery disease and peripheral vascular disease, are caused by the build-up of fatty atherosclerotic deposits (plaque) in the arteries. These deposits limit blood flow to the tissues that are supplied by that particular artery. Risk factors for this type of disease include advanced age, diabetes, high blood pressure, obesity, history of smoking, and high cholesterol or triglycerides.
When these deposits build up in the arteries of the heart, the problem is called coronary artery disease (CAD). When these deposits build up in the arteries of a limb, such as a leg, the condition is called peripheral artery disease (PAD). Symptoms of CAD—angina, heart disease, and heart attacks, are well known. Symptoms of PAD can include pain on walking, and wounds that do not heal. If PAD is not treated, it can eventually produce critical limb ischemia (CLI), gangrene, and loss of limb. Roughly 30% of the population over the age of 70 suffers from PAD.
When the plaque builds up to the point where an artery is totally occluded, the obstruction is referred to as a Chronic Total Occlusion (CTO). CTOs can confound the treatment of CAD, because the sudden loss of heart muscle can lead to sudden death. A CTO that occludes the peripheral arteries for PAD patients is also extremely serious. PAD patients that suffer from a CTO often enter a downward spiral towards death. Often the CTO in a peripheral artery results in limb gangrene, which requires limb amputation to resolve. The limb amputation in turn causes other complications, and roughly half of all PAD patients die within two years of a limb amputation.
For both CAD and advanced PAD, prompt treatment of such blockages is thus essential. Here, less invasive angioplasty or atherectomy procedures have many advantages. In these procedures, a catheter is inserted into the diseased artery and threaded to the blocked region. There the blockage may be either squeezed into a hopefully more open position by pressure from an inflated catheter balloon (balloon angioplasty), the blocked region may be kept open by a stent, or alternatively a physician may use a catheter to surgically remove the plaque from the inside of the artery (atherectomy).
As an example, for the treatment of PAD, atherectomy devices such as the Fox Hollow (now ev3) SilverHawk™ catheter (U.S. Pat. No. 6,027,514), are often used. These catheters may be threaded (usually with the aid of a guidewire) up the artery to a blocked region. There, the physician will usually position the catheter to make multiple passes through the blocked region of the artery, each time shaving a way a ribbon of plaque. The shaved ribbons of plaque are stored in the hollow nose of the device. By making multiple passes, the plaque may be substantially reduced, blood circulation may be restored to the limb, and the limb in turn saved from amputation.
In order to effectively treat the plaque, however, most modern catheters need to be threaded past the blocked region of the artery. This is because the active portions of most catheters, which are used to treat the blockage, are usually located on the side of the catheter, rather than on the tip of the catheter. This is due to simple mechanical necessity. The tip of the catheter must have a very small surface area, and thus is able to treat only a very small portion of the diseased artery. By contrast, the side of the catheter has a much larger surface area, and the catheter side thus conforms nicely to the sides of the diseased artery. Thus stents, balloons, atherectomy cutting tools, etc., are usually mounted on the sides of the catheter. The catheter must be threaded past the blocked portion of the artery in order to function properly.
When the artery is only partially blocked by plaque, the catheter can usually be maneuvered past the obstruction, and the active portions of the catheter can thus be brought into contact with the diseased portion of the artery. However when the artery is totally blocked, as is the case with a CTO, this option is no longer possible. The tip of the catheter encounters the obstruction, and further forward motion is blocked.
Simply trying to force a typical catheter past the obstruction usually isn't possible. The obstructions are typically composed of relatively tough fibrous material, which often also includes hard calcium deposits as well. Often, when physicians attempt to force guidewires or catheters past such obstructions, the guidewire or catheter device may instead exit the artery and enter the lumen outside the artery. This further damages the artery, further complicates the procedure, and decreases the chance of success. As previously discussed, the consequences of such procedure failures have a high mortality rate. Thus improved methods to allow catheters and guidewires to more readily penetrate through hardened plaque and CTO are thus of high medical importance.
A good summary of the present state of the art may be found in an article by Aziz and Ramsdale, “Chronic total occlusions—a stiff challenge requiring a major breakthrough: is there light at the end of the tunnel?” Heart 2005; 91; 42-48.
Previous attempts to produce devices for cutting through hardened plaque include U.S. Pat. No. 5,556,405 to Lary, U.S. Pat. No. 6,152,938 to Curry, and U.S. Pat. No. 6,730,063 to Delaney et al.
U.S. Pat. No. 5,556,405 teaches an incisor catheter which features a bladed head stored in a catheter housing, which contains a number of slits though which the blades protrude. The blade is activated by a push-pull catheter. When the push-pull catheter is pushed, the bladed head protrudes through the slits in the housing, and the blade thus comes into contact with hardened plaque material. The blade does not rotate, but rather delivers linear cuts.
U.S. Pat. No. 6,152,938 teaches a general purpose catheter drilling device for opening a wide variety of different blocked (occluded) tubes. The device anchors the tip of the drill head against a face of the occlusion, and partially rotates the drill head using a rein attached to the drill head so that the drill head faces at an angle.
U.S. Pat. No. 6,730,063 teaches a catheter device for chemically treating calcified vascular occlusions. The device is a fluid delivery catheter that delivers acidic solutions and other fluids to calcified plaque with the objective of chemically dissolving the calcified material.
Several catheter devices for traversing CTO obstructions are presently marketed by Cordis Corporation, FlowCardia Technology, Kensey Nash Corporation, and other companies. Cordis Corporation, a Johnson and Johnson Company, produces the Frontrunner® XP CTO catheter (formerly produced by LuMend Corporation). This catheter, discussed in U.S. Pat. No. 6,800,085 and other patents, has a front “jaw” that opens and closes as it traverses the catheter. The jaw itself does not cut, but rather attempts to pry open the CTO as the catheter passes.
Other catheter devices use various forms of directed energy to traverse CTOs. For example, FlowCardia Technology, Sunnyvale Calif., produces the Crosser system, taught in U.S. Pat. No. 7,297,131 and other patents. This system uses an ultrasonic transducer to deliver energy to a non-cutting catheter head. This catheter head itself has a relatively small diameter and does not have any blades. Rather, the head, through rapid (ultrasonic) vibration is able to push its way through a variety of different occlusions.
Kensey Nash Corporation, Exton Pa. (formerly Intraluminal Therapeutics, Inc.), produces the Safe-Cross CTO system. This system, taught in U.S. Pat. Nos. 6,852,109 and 7,288,087, uses radiofrequency (RF) energy. The catheter itself is also directed in its movement by an optical (near-infrared light) sensor which can sense when the tip of the catheter is near the wall of the artery. The optical sensor tells the operator how to steer the catheter, and the RF ablation unit helps the operator ablate material and cross occluded regions.
Although ingenious, the success rates with these devices still leave much to be desired. According to Aziz, the best reported success rates of overcoming CTOs with prior art devices range from 56% to 75%. Aziz further teaches that the average success rates are only in the 50-60% range. Given the huge negative impact that unsuccessfully cleared CTO's have on patient morbidity and mortality, clearly further improvement is desirable. An additional problem with these prior art CTO clearing devices is that simply cutting a small channel though the CTO may not be sufficient to totally resolve the medical problem. Occasionally, the device that traverses the CTO should also remove (debulk) a substantial portion of the occlusion. This is because as previously discussed, removal of a substantial portion of the occlusion may be required in order to allow catheters with side mounted stents, balloons, and atherectomy cutting tools to get access to the damaged portions of the artery and make more lasting repairs. Thus improved CTO “unclogging” devices that can do the more substantial amount of CTO debulking required to allow other types of catheters to pass are also desirable.
Thus there remains a need for devices that can effectively traverse CTOs and remove more substantial amounts of hardened or calcified plaque. Such devices would enable stents and other devices, such as SilverHawk atherectomy catheters, balloon catheters, etc. to be more successfully used in high occlusion situations. This in turn should lead to improved patient outcomes and a reduction in patient morbidity and mortality.